eMedicine Specialties > Radiology > Pediatrics

Ureteropelvic Junction Obstruction, Congenital

John S Wiener, MD, FACS, FAAP, Clinical Assistant Professor, Division of Urology, University of North Carolina at Chapel Hill; Associate Professor of Surgery and Associate Residency Program Director, Division of Urologic Surgery, Associate Professor of Pediatrics, Duke University School of Medicine
Benjamin K Yang, MD, Urologist, Riverside Medical Clinic; Anamaria Gaca, MD, Clinical Instructor, Division of Pediatric Radiology, Duke University Medical Center
Contributor Information and Disclosures

Updated: Dec 30, 2008

Introduction

Background

Congenital ureteropelvic junction (UPJ) obstruction is the most common cause of upper urinary tract obstruction in children. By definition, this diagnosis signifies functionally impaired transport of urine from the renal pelvis into the ureter. Because the increased renal pelvic pressure from obstruction may lead to progressive renal injury and embarrassment, correct diagnosis is clinically important. The impairment may be primary or secondary in nature. This, along with the chronicity and severity of the condition, dictates the course of management. Radiologic imaging is crucial in diagnosing UPJ obstruction.^{1, 2, 3, 4}

For excellent patient education resources, visit eMedicine's Kidneys and Urinary System Center. Also, see eMedicine's patient education article Intravenous Pyelogram.

Pathophysiology

Intrinsic narrowing of the junction of the renal pelvis and ureter is the most common cause of primary congenital ureteropelvic junction (UPJ) obstruction. This is typically the result of a patent but aperistaltic segment at the UPJ. Normally, urine is transported from the pelvis into the ureter by peristalsis initiated by electrical impulses from primary pacemaker cells in the calyces. The impulses generate a contraction wave that propagates from the pelvis to the ureter, which acts as a functional syncytium.

With UPJ obstruction, a functional obstruction occurs as this contraction wave is interrupted, and active urine transport across the UPJ is impaired. On inspection, these obstructing segments are often short and are narrower in caliber than the adjoining ureter. Microscopically, they demonstrate a decreased quantity of muscle fibers, which are often replaced by fibrotic tissue, as well as a distortion of the normal spiral orientation of the fibers. Therefore, obstruction is likely the result of functional (impaired muscle activity) and anatomic (reduced luminal diameter) causes.

Abnormal recanalization of the ureter during embryologic development is the favored hypothesis as to why congenital UPJ obstruction occurs. In normal human development, the ureter begins as a tube lined with simple cuboidal epithelium; it acquires a complete lumen at 28 days' gestation. At about 37-40 days, transient luminal obliteration occurs, followed by recanalization through apoptosis. Recanalization begins in the midureter and proceeds in cranial and caudal directions. A failure of the most proximal region to recanalize normally could, therefore, lead to congenital obstruction at the UPJ. Other, less common intrinsic causes for congenital UPJ obstruction include the development of atypical mucosal valves, polyps, and true ureteral strictures.

The most common extrinsic cause of UPJ obstruction is mechanical obstruction associated with aberrant or supernumerary renal vessels crossing closely around the UPJ. In a significantly high number of patients with UPJ (about 33%), an artery directly enters the lower pole of the kidney.⁵ Inspection often demonstrates kinking of the ureter under the vessel. Histologically, features of an adynamic segment of ureter are observed. Whether compressive forces induce these changes or whether these are separate entities that additively cause the obstruction is unknown. The presence of a crossing vessel necessitates the correction of the obstructed UPJ segment, as well as the transposition of the UPJ to the side opposite the vessel.

Secondary congenital UPJ obstruction may result from severe vesicoureteral reflux (VUR). VUR may lead to ureteral elongation, kinking, and, eventually, mechanical obstruction at the UPJ.

Congenital UPJ obstruction may result in a spectrum of altered renal functions in the hydronephrotic kidney. The degree of functional impairment in a congenitally obstructed kidney depends on the timing of the onset of obstruction and its severity. Early and total obstruction, which is rare, leads to a completely nonfunctional kidney, as seen in renal dysplasia. Late and partial prenatal obstruction leads to varying degrees of caliectasis and functional renal impairment.

In the obstructed kidney, the initial pathologic response is an elevation in renal pelvic pressures. Following this, there is a characteristic compensatory reduction in renal blood flow and glomerular filtration rate (GFR) to bring pelvic pressures back into the normal range. Key animal studies have demonstrated that the mode of injury from obstruction is likely ischemic injury; such injury occurs as a result of this chronic compensatory reduction in renal blood flow, not as a direct effect of elevated pressures on cellular development.^{6, 7} This deleterious effect of obstruction may be further amplified in the developing fetal kidney.

On the basis of observational studies regarding the natural history of congenital UPJ obstruction, about 25% of hydronephrotic newborn kidneys will demonstrate clinical or functional deterioration warranting surgical repair. However, in most cases, renal function will be preserved. Although surgery does not uniformly result in an improvement in renal function, some patients experience an increase in differential renal function, and a further reduction in function is prevented in the remainder of patients. A certain percentage of kidneys with UPJ obstruction are larger in size than the contralateral kidney, possibly because of hyperfiltration; in such cases, the enlarged kidney may have greater function than the normal kidney. In general, early surgical correction in cases of definite obstruction increases the potential for postoperative improvements in renal function and growth; some report that the optimal period for performing surgery is at 3-6 months of age.

Frequency

United States

With the advent of prenatal ultrasonography, most cases of ureteropelvic junction obstruction are now detected in utero. In utero hydronephrosis is found in 1 in 100 pregnancies; in most of these cases, the hydronephrosis resolves. Clinically significant neonatal hydronephrosis occurs in 1 in 500 pregnancies, with ureteropelvic (UPJ) obstruction accounting for about 40-50%. UPJ is by far the most common cause of pediatric hydronephrosis.

International

Internationally, the incidence of ureteropelvic junction obstruction does not appear to differ significantly from that reported in the United States.

Mortality/Morbidity

The major morbidity associated with functional ureteropelvic junction obstruction is progressive renal injury resulting from relative ischemia, caused by the compensatory response of decreased renal blood flow and decreased glomerular filtration rate (GFR). In the long term, this may lead to glomerular sclerosis and proteinuria, as well as hypertension and its related sequelae. Chronic renal failure may occur if there is contralateral renal impairment.

• Only about 1 in 4 neonates with prenatally diagnosed severe hydronephrosis will have progressive worsening of renal function, hydronephrosis, or symptoms requiring surgery to prevent further damage.^{8, 9}
• Other morbidities associated with UPJ obstruction include urinary stasis leading to urinary tract infections (UTIs) and stones; flank pain; nausea and vomiting; and hematuria. Recurrent UTIs in this setting are particularly troublesome because they can further damage the kidney.

Race

The incidence of ureteropelvic junction obstruction has not been shown to be higher incidence in any particular race.

Sex

Ureteropelvic junction obstruction is more common in males than in females by a ratio of 2:1.

Age

Hydronephrosis has increasingly become a prenatal diagnosis with the advent of prenatal ultrasonography.

• Before prenatal screening, about 25% of cases were diagnosed during the first year of life, with symptomatic presentation.
• Beyond the first year, congenital UPJ obstruction is seen with decreasing frequency.
• See also Clinical Details, below.

Anatomy

The ureteropelvic (UPJ) junction is an anatomically indistinct segment of the upper collecting system where the renal pelvis funnels into the ureter. In normal anatomy, the renal pelvis exits the kidney at the renal hilum posterior to the renal vein and artery. In 25-40% of kidneys, a supernumerary artery crosses the urinary collecting system on its course into the lower pole of the kidney; this may cause extrinsic mechanical obstruction. The blood supply for the UPJ itself is typically derived from branches of the renal artery and is anteromedial to the ureter.

The UPJ, like the rest of the ureter, consists of 2 layers of smooth muscle: an inner longitudinal layer and an outer circular, or spiral, layer. This muscle actively propels urine through the lumen.

Presentation

Infants are usually asymptomatic; in these patients, ureteropelvic junction obstruction is diagnosed through prenatal screening ultrasonography. In contrast, older children and adults present typically with symptoms of episodic flank or abdominal pain (seen in 50% of cases); a palpable flank mass (50%); recurrent UTIs (30%); and nausea and/or vomiting. Infants who did not undergo prenatal screening or those who had been lost to follow-up may present with a palpable mass, feeding difficulties, or failure to thrive secondary to nausea and/or vomiting, hematuria, pain, or urosepsis. In infancy, hydronephrosis is the most common cause of a palpable abdominal mass. Older patients may present with gross hematuria following mild abdominal trauma, owing to disruption of mucosal vessels in the dilated collecting system.

Rarely, a patient may present with long-standing, renin-mediated hypertension induced by reduced blood flow to the obstructed kidney. Other patients may present asymptomatically; in these patients, UPJ obstruction is found incidentally on radiologic studies performed for other reasons.

UPJ obstruction is often associated with other congenital renal anomalies, including ectopic or horseshoe kidneys; duplication of the collecting system, usually affecting the lower half; contralateral renal dysplasia; multicystic dysplastic kidney (MCDK); or renal agenesis. The presence of these anomalies should increase the index of suspicion for UPJ obstruction, especially if the clinical picture is correlated.

As many as 50% of patients with UPJ obstruction have other urologic abnormalities, including contralateral UPJ obstruction and VUR (<40%), which is often low grade and likely to resolve spontaneously. Abnormalities in other organ systems also are frequently (albeit sporadically) seen. About 21% of children with VATER complex (vertebral defects, anal atresia, tracheoesophageal fistula with esophageal atresia, and radial and renal dysplasia) have a UPJ obstruction.

In the era of prenatal ultrasonography, the challenge of addressing congenital UPJ obstruction is in distinguishing cases of nonobstructing hydronephrosis that will resolve spontaneously or remain stable from those that will lead to progressive renal destruction.¹⁰

UPJ obstruction occurs more often on the left side than on the right by a ratio of 3:2. Bilateral obstruction occurs in 10-40% of patients.

Preferred Examination

The widespread practice of prenatal ultrasonography has made ultrasonographic findings the primary presentation of congenital ureteropelvic (UPJ) obstruction. Routine prenatal assessment typically occurs at 16-20 weeks' gestation. On the basis of those findings, an appropriate series of in utero and postnatal studies is performed.

Current radiologic standards define hydronephrotic kidneys as those with an anteroposterior (AP) diameter at the renal pelvis of greater than 4 mm at a gestational age of less than 33 weeks and an AP diameter of greater than 7 mm at a gestational age of 33 weeks or older. An abnormal initial ultrasonogram should be followed up with another ultrasonogram after 4 weeks in severe cases or after 33-34 weeks in mild to moderate cases.

The most common criteria used to grade fetal hydronephrosis are the Society of Fetal Urology (SFU) consensus guidelines, which are based on pelvic dilation and caliectasis. They include the following:

• Grade 0 — Normal kidney
• Grade 1 — Minimal pelvic dilation
• Grade 2 — Greater pelvic dilation without caliectasis
• Grade 3 — Pelviectasis and caliectasis without cortical thinning
• Grade 4 — Hydronephrosis with cortical thinning

Grade 3-4 hydronephrosis is 88% sensitive and 95% specific for obstruction on diuretic renograms (see Images below and Images 1-3 in Multimedia). However, because of interobserver and intraobserver variability in assessing the degree of dilatation in ultrasonography, many centers continue to rely on the AP diameter of the renal pelvis to diagnose hydronephrosis. The hydration status of patients naturally has a bearing on the measurement and assessment of dilatation in the collecting system, and any extreme outlying states should be duly noted.

Much controversy exists over the optimum timing of follow-up postnatal imaging. Some suggest delaying imaging at least 48 hours after birth to minimize false-negative findings, owing to the infant's relative state of dehydration and decreased GFR. Others have found no difference between early and delayed ultrasonography. This issue is of some practical significance because an early ultrasonogram may be obtained before the infant is discharged home from the nursery, which helps prevent noncompliance with follow-up.

Typically, disease of the more severe grades requires routine serial monitoring every 3-4 months for the first year of life; hydronephrosis of grades 1 and 2 may be monitored with less frequency (every 6-9 months). For hydronephrosis of grades 3 and 4, administering a diuretic nuclear renogram to assess renal function is usually delayed until 1 month of age to allow for the physiologic increase and stabilization of GFR. The need for repeat nuclear renography is controversial; the decision is made large on the basis of physician preference, ultrasonographic findings, or both. Definitive treatment is initiated with the finding of reduced or worsening renal function, worsening hydronephrosis, or both. Because reflux may cause hydronephrosis and coexists in 13-42% cases of congenital UPJ obstruction, cystourethrography is performed for all patients with these findings to assess VUR.

Not uncommonly, asymptomatic UPJ obstruction is discovered in older children or adults when radiologic studies, such as ultrasonography, intravenous pyelography (IVP), computed tomography (CT) scanning, or magnetic resonance imaging (MRI), are performed for other reasons. Often, the initial examination is a CT scan, because CT has become the study of choice at many institutions for evaluating acute flank or abdominal pain. The use of intravenous contrast material—with nephrogenic and delayed excretion phases—during CT scanning may provide qualitative information regarding obstruction, but in general, CT should be avoided because of the inherent high radiation dose. When hydronephrosis is seen when UPJ obstruction is suspected, diuretic renography is more accurate than CT.

IVP has historically been the primary study used to diagnose UPJ obstruction in adults, because it also provides anatomic and functional information. However, ultrasonography is preferred as the initial study in children because of its nonionizing and noninvasive nature. Once an obstruction is suspected on the basis of anatomic studies, the diagnosis of UPJ obstruction is made by means of diuretic renography.

Limitations of Techniques

To diagnose ureteropelvic junction obstruction, anatomic and functional studies are necessary. As a stand-alone study, ultrasonography cannot definitely depict UPJ obstruction. Ultrasonography provides no information regarding physiologic or functional status, and in many cases, despite the presence of hydronephrosis, urine passage is unobstructed. Obstruction must be demonstrated by means of functional studies, such as diuretic nuclear renography. Even with nuclear renography, the definition of true obstruction is a subject of debate; some proponents rely on decreased differential renal function as an indication of obstruction, whereas others rely on a decrease in washout time. Many features of the infant kidney, including its immature state and its variable appearance (depending on volume status), may lead to inaccurate ultrasonographic findings.

Differential Diagnoses

Other Problems to Be Considered

Prenatal conditions

Nonobstructive hydronephrosis
VUR
Ureterovesical junction (UVJ) obstruction
Posterior urethral valves (PUV)
MCDK
Duplication anomalies
Prune belly syndrome
Intestinal disorders — intestinal duplication, mesenteric cysts, anal atresia, persistent cloaca, cloacal exstrophy
Ovarian cysts
Neuroblastoma
Congenital mesoblastic nephroma

Pediatric conditions

Nonobstructive hydronephrosis
Sepsis
UVJ obstruction
PUV
MCDK
Neurogenic bladder
Prune belly syndrome
Solid renal tumors — Wilms tumor, neuroblastoma, mesoblastic nephroma, renal cell carcinoma
Renal vein thrombosis
Pyelonephritis
Hypoechoic immature medulla in neonates and infants

References

• Kidneys and Urinary Systems Center
• Intravenous Pyelogram Introduction
• Intravenous Pyelogram Preparation